A hot-swap circuit applies power from an input source to a load in a controlled and protected fashion. One function of such a controller is to limit inrush currents from the power source to the load, especially load capacitance, when power is first applied or if the power source voltage suddenly increases. Another function is to limit current if the load attempts to draw too much current, for example if there is a short circuit in the load.
FIG. 1 shows a conventional hot-swap circuit having an input node VIN for receiving power from a power source, and an output node Vout coupled to a load. A single MOSFET 100 is coupled in series with a current sense resistor 102 between the input node VIN and the output node VOUT. The hot swap circuit also has control circuitry including a voltage source 104, a current limit amplifier 106, a current source 108 and a transistor 110.
Numerous such circuits are commercially available. When limiting current, a current limit amplifier 106 compares a voltage representing the current in the current sense resistor 102 with a voltage VLIMIT produced by the voltage source 104 to control the gate of the MOSFET 100 so as to reduce the current flowing through the MOSFET 100 when the sensed current exceeds a maximum value established by the voltage VLIMIT. The current limit amplifier 104 adjusts the gate to source voltage of the MOSFET 100 in order to limit the voltage across the current sense resistor 102 and thus the current through the MOSFET 100. The current source 108 is provided for pulling up the gate voltage. A transistor 110 is provided for supplying the MOSFET 100 with ON and OFF signals to command the MOSFET 100 to turn on or off, respectively.
During a current limit operation, the voltage and current through the MOSFET 100 can both be large, resulting in high power dissipation in the MOSFET 100. If this power dissipation persists, the MOSFET 100 can reach temperatures that cause damage. MOSFET manufacturers present the safe limits on MOSFET voltage, current, and time as a curve referred to as Safe Operating Area (SOA). Commonly, a timer circuit 112 sets a maximum time period during which the MOSFET 100 is allowed to operate in a current limit mode.
The timer circuit 112 may be coupled to the current limit amplifier 106 for receiving a signal indicating that the current limit operation is initiated. When the time period set by the timer circuit 112 expires, an overcurrent fault signal is produced, and the MOSFET 100 may be turned off to protect it from overheating. The load will lose power and the hot swap controller will indicate that a fault has occurred.
Often high power hot-swap applications need to charge large bypass capacitors 126 (CL) across the load. To reduce stress on the MOSFET 100, the load may be kept off until the bypass capacitors 126 are charged. A small charging current for the capacitance keeps the power in the MOSFET 100 low enough to prevent a dangerous rise in temperature. The gate voltage may be pulled up by a current from the current source 108 commonly in the range of 10-50 μA.
Hot swap controllers also commonly generate a signal to indicate that power is good and the load can safely draw current. For example, a power good signal may be produced by a comparator that monitors the output voltage. Power is considered good when the output voltage has risen above a threshold. The output voltage may be used as the condition for indicating that the power is good.
Also, a power good signal may be produced by monitoring the switch turn-on control signal. For a MOSFET switch 100 this is the gate to source voltage. If it is significantly above the MOSFET threshold voltage, then the MOSFET channel is fully on and load current may flow through it. However, this gate to source voltage may be reduced during brief episodes of current limit during normal operation. In such situations, the output power still may be considered good. Thus, the signal indicating that the gate to source voltage has exceeded a threshold is latched. This latched signal is used to indicate that the MOSFET has turned fully on and the load may be turned on. The latched signal will continue to indicate that power is good even if the gate voltage is subsequently reduced during short duration current limiting events. The latch is reset if the MOSFET is ever turned off.
Further, a power good signal may be produced by monitoring a voltage between drain and source of the MOSFET 100. Once this voltage has gone below a threshold, then the MOSFET 100 is assumed to be fully on and load current may flow through it. However, the voltage between drain and source may also increase during normal operation, for example if the input voltage increases quickly. In such situations, output power still may be considered good. Thus, the signal indicating that the voltage between drain and source is below a threshold can also be latched. This latched signal can be used to indicate that the MOSFET 100 has turned fully on and the load may be turned on.
In a hot-swap application, several things can prevent the MOSFETs from turning on with low impedance. A damaged MOSFET may have leakage from gate to drain or have degraded drain to source on-resistance RDS(ON). Debris on the board may also produce leakage or a short from the gate pin to the source pin, the MOSFET drain, or to ground. In these conditions the hot-swap controller may not be able to pull the gate pin high enough to fully enhance the MOSFET, or the MOSFET may not reach the intended on-resistance when the gate pin is fully enhanced. This can put the MOSFET in a condition where the power in the MOSFET is higher than its continuous power capability, even though the current is below the current limit.
Conventional methods determine that the MOSFET has fully turned on by monitoring gate to drain or gate to source voltage. That information is then latched. However, if the MOSFET subsequently degrades in performance, the latched information will not be updated to indicate that a problem has developed.